Magneto-rheological suspensions include dispersions of ferrous and/or ferric particles suspended throughout a carrier matrix. Examples of carrier matrices include hydrocarbon oil, mineral oil, silicon oil, and grease, among others. The particles generally remain suspended throughout the carrier matrix and are often randomly dispersed when the suspension experiences flow. When influenced by a magnetic field, the particles become polarized and are attracted to each other, often forming particle chains that align with the magnetic field. These structures result in an increased apparent viscosity of the suspension that can be proportional to the strength of the applied magnetic field up to a particular strength limit of the applied magnetic field. Above the strength limit, magnetic permeability of the magneto-rheological suspension can become saturated and the apparent viscosity may no longer vary in proportion to the strength of the applied magnetic field.
Demand for miniaturized magneto-rheological devices that utilize magneto-rheological suspensions is increasing. Magneto-rheological suspensions can be used in combination with micro-electro-mechanical systems (MEMS) devices, complex micro-fluid handling systems, control devices for small structures, and cooling devices of macro-scale components. Other examples include small-scale controllable dampers (e.g., shock absorbers) that incorporate microchannels, rotary brakes, and fluid clutches. Alternatively, magneto-rheological suspensions may be used in combination with microvalves, as disclosed in Provisional U.S. Patent Application 60/347,928, filed Oct. 17, 2001, incorporated by reference herein in its entirety, and as disclosed in Provisional U.S. Patent Application 60/334,989, filed Oct. 23, 2001, incorporated by reference herein in its entirety.
Magneto-rheological suspensions can be desirable for use in these devices because magneto-rheological suspensions exhibit variable bulk properties that can be controlled through exposure to electric and/or magnetic fields. For example, magneto-rheological suspensions that exhibit increased apparent viscosity when influenced by a magnetic field can be applied to dampen mechanical energy at variable rates. Future development of miniaturized devices that employ magneto-rheological suspensions requires knowledge of operating characteristics of magneto-rheological suspensions.